This Recommendation, entitled "Frame structure for a 64 kbit/channel in audio visual teleservices", describes how to organize data to be transmitted for audio and video teleservices on a single 64 kbit/s channel, wherein information encoding algorithms, frame structure and already existing CCITT Recommendations are optimized.
The 64 kbit/s channel is structured into octets transmitted at 8 kHz, wherein the eighth bit of every octet is used to form an 8 kbit/s subchannel, referred to as service channel, whereupon the signalling is transmitted and the remaining 7 bits, with 56 kbit/s transmission capacity, carry a variety of signals, among which are speech signals, data and speech signals, still pictures, data in an audio visual session, etc.
A succession of 80 octets forms a frame, containing the information necessary for the receiver to recover timing, and 16 successive frames form a multiframe, still containing timing information, properly allocated in the service channel. At the frame level a primary-importance information is transmitted; it is protected by error correcting codes which correct up to two errors and signal a third error. This information concerns data and frame structure organisation, which can change even during the transmission. There is another error signalling method, based on cyclic redundancy code (CRC), which detects the presence of errors in all the bits of the frame, but does not perform corrections. It is mainly used to evaluate transmission quality.
Prior to this protocol, CCITT had defined an analogous protocol identified by Y221, which had been implemented by an equipment designed within an ESPRIT project and presented in the paper entitled "MIAC-ESPRIT PROJECT 1057 FOR MULTIPOINT INTERACTIVE AUDIOVISUAL COMMUNICATION", issued in ESPRIT Technical Week, Brussels, 1988. However no information has been issued on the circuits contained in the apparatus.
The H221 and Y221 protocols differ from each other on a number of points, e.g. in the way of transmitting the information relevant to data organization within the frame, namely as regards the way of correcting possible transmission errors.